Hooking Computers Together
If you wanted to go to the trouble of setting up a SSH into the Windows computers, you could control them over the network from your Mac. The problem is that it takes a lot of work to set up a Unix-Windows SSH. You can find resources for doing that here. If you are comfortable with the working in the Unix shell and do get that set up, then you can put together a &quot;pseudo&quot; network render that doesn't need the Blender Network Render. I use a similar method to render on Linux servers since they don't have a GUI.

Install Dropbox on the Windows computers. Make sure that you know where the Dropbox folder is.
In the .blend file you want to render, under the Output panel in the Render settings, uncheck &quot;Overwrite&quot; and check &quot;Placeholders&quot;.
Upload the .blend file to Dropbox, so that it automatically is copied to both of the Windows computers.
Run Blender on each of the Windows computers from the SSH shell. You can use the command &lt;path to blender&gt; -b &lt;path to file&gt; -o &lt;path to Dropbox&gt; to run Blender in the background and render whatever file you specify. Both of the Windows computers will render whatever frames you specified in the .blend file to the Dropbox folder.

The advantages of this method are:

You don't have to use the network renderer (no &quot;Slave&quot; and &quot;Master&quot; configuration to worry about)
It is really easy to add in another computer: just install Dropbox, SSH in, and run the command.
You can see exactly where each of the working computers are from the terminal on your Mac.

Disadvantages:

It requires a basic knowledge of the Unix shell (command line)
Could possibly be an extra bit of a pain since the Windows computers would have to have extra programs installed to be able to communicate in that way with the Mac.
Every time you cancel mid-render, each computer will leave a blank image file (the one it was in the middle of rendering). This isn't a big deal as long as you remember to delete it, so that the computers will pick up where they left off.

Basically, there isn't a simple way to do what you want. In spite of that, it is possible. Hope that this helps!